The present invention relates generally to water treatment, and more particularly to methods and apparatus for treating so-called "white-water" used in paper processing so as to reduce the biochemical or biological oxygen demand (B.O.D.) and also lower the concentration of dissolved organic solids in the white water. As is well known, the manufacture of paperboard and other paper products requires a great deal of water.
Typically, in the paperboard manufacture, waste paper products are pulped and refined, and then added to water to make a fiber slurry. The slurry is fed to one or more cylinder chests wherein the fibers are picked up from the slurry. The fibers are then transferred from the cylinders to a continuously moving screen, with each cylinder adding enough fibers to make an individual ply. Thus, a multiple-ply paper product is deposited and then removed from the screen after being passed through a series of rollers which squeeze the wet product to remove excess water therefrom.
Because of the increasing concern over water pollution, the increased cost of water, and the rapidly increasing cost of energy, there has been an increased interest in methods and apparatus for more efficient paper manufacture and processing.
Referring now to one problem in the paper industry, if white-water, particularly white-water used in the manufacture of paper from recycled paper products such as newsprint, kraft paper, paperboard and other box cuttings, etc., is discharged into open waters, serious environmental damage results because the white water has a high biological oxygen demand.
Oxidation of white water products depletes available oxygen in the water below desirable levels, damaging or threatening marine life supported by the ecosystem of which the stream or pond forms a part.
It is possible to avoid this problem by creating large treatment ponds or the like wherein the white water is stored until the solids dispersed therein settle out, and wherein the oxygen demand is eventually satisfied from the air or elsewhere from the treatment tank area. Because such ponds may be specially provided, damage to other water is avoided. However, the white water treatment pond approach is seriously disadvantageous in that it is time consuming, requires significant space, often creates highly obnoxious odors, and very importantly today, is highly wasteful of energy.
The energy waste involved in the reuse of treated white water comes from the requirement of reheating the clarified water up to mill use temperatures after it has been allowed to cool to ambient temperatures over a period of several days or more. With the high cost of gas, oil, or coal, the cost of lost heat is not only enormous, but is essentially a loss without offsetting advantages.
If the white water is reused without treatment, the solids content thereof builds up to the point where machinery becomes clogged and the paper product becomes damaged by discoloration, obnoxious odor, or both. The problem of solids buildup in white-water is a recurring problem not only because of the wide variety of recyclable papers fed to the stock chest, but also because the nature of the feed material is more or less uncontrolled. Examples of the materials which are present in the white-water resulting from recycling paper are carrier oils, the vehicle components of newsprint inks, residual lignins and other binders from recycled paper, starch and related decomposition products of cellulose, gums and perhaps other natural dispersants from paper adhesives, wet strength resins of various kinds, including ketene dimers, for example, and a wide variety of other organic materials. Moreover, there are many inorganic materials, such as carbonates, clays, and the like, a number and variety of which may be surface active and resist settling or separation by usual methods.
In addition to the problem of odor, white water with the above contaminants, particularly in the presence of bacteria, can become extremely corrosive, and, if not neutralized, will rapidly damage pipes, valves, and other elements of the paper processing equipment.
According to the invention, an apparatus for exposing the white-water to ultraviolet radiation makes possible the reduction of both dissolved solid materials and biological oxygen demand. Although the exact mechanism of the invention is not understood in detail, and although the invention is not limited to any particular theory of operation, it is believed that the exposure of the water to ultraviolet light serves to polymerize the simpler components of the materials in solution, rendering them insoluble and capable of ready separation. According to the invention, the process of separation can be carried out continuously so that the white water may be used over and over, with fibers being added periodically and impurities being removed continually, without creation of a process atmosphere which is characterized by corrosion, by obnoxious odors, or by pollution of outside waterways, no need for treatment lagoons, and the temperature drop and hence energy loss of the white water during processing is minimal.
In view of the shortcomings of the prior art methods of treating white water used in paper processing, it is an object of the invention to provide an improved method of processing papermaking white water.
Another object of the invention is to provide a method of treating white water which can be carried on continuously during the papermaking process and which does not require removal of white water periodically for processing in treatment lagoons.
A still further object is to provide a white water treatment method which is adaptable for use with existing paper processing equipment and which may be carried out continuously on the white water so as to render the white water reusable indefinitely in the papermaking process.
Another object is to provide a method of treating papermaking white water which cost as little or less than prior art treatments exclusive of the heat savings able to be achieved.
Still another object is to provide a white water treating method which can be performed in an apparatus which can be readily added to existing paper processing equipment and which does not require changes to be made in such equipment. Another object is to provide a method of white water treatment which includes the step of treating the white water with ultraviolet light to induce polymerization of dissolved, polymerizable monomers into higher molecular weight, perhaps highly branched compounds, of decreased solubility so as to render the resulting compounds capable of ready separation from the white water.
Yet another object is to provide a method of white water treatment which permits white water to be treated at ambient temperatures characteristic of the papermaking process, thus eliminating the need for cooling and reheating the white water.
A still further object of the invention is to provide a method of reducing the biochemical or biological oxygen demand of papermaking white water so as to reduce the potential for creating an anaerobic atmosphere which is conducive to the growth of anaerobic bacteria, which in turn create highly corrosive, odiferous organic products, such as hydrogent sulfide, butyric acid, and other such compounds.
Another object is to provide a method of treating papermaking white water so as to minimize the formation of by-products, particularly short chain organic acids, which tend to impart undesirable odors to the finished paper products.
A still further object is to provide a method of treating papermaking white water which eliminates or reduces the buildup of dissolved organic solids which tend, particularly when the process equipment is shut down, to clog or foul process plumbing and fiber mats or screens on which the paper is laid.
Another object is to provide a method of papermaking white water treatment which enables the white water to be reused indefinitely, with the method involving the continuous maintenance of acceptable minimum level of dissolved organic solids and an acceptably reduced biological oxygen demand in the white water.
A still further object is to provide a method of treating papermaking white water which includes treating the water by withdrawing portions thereof from the system from time to time, thereby permitting the volume of treated water to be independent of the total volume being used by the mill at any given time.
Another object is to provide a method of treating papermaking white water in which the water may be treated periodically or intermittently, whether on a sampling basis or otherwise, and still achieve the intended result of reducing dissolved organic solids and biological oxygen demand.
A still further object is to provide a method of treating white water which will eliminate or reduce biological induced corrosion of the process equipment used in papermaking.
These and other objects and advantages of the invention are achieved in practice by treating the process of white water at process temperatures with ultraviolet light of a preferred frequency so as to polymerize dissolved materials and to separate the resulting materials from the white water intermittently or continuously during the papermaking process.
The manner in which these and other objects and advantages of the invention are achieved in practice will become more clearly apparent when reference is made to the accompanying detailed description of the preferred manner of practicing the invention set forth by way of example and illustrated in the accompanying drawings, wherein like reference numbers indicate corresponding parts throughout.